1. Field of the Invention
The present invention relates to a ceramic circuit substrate and a method of producing the ceramic circuit substrate. More specifically the present invention relates to adhesion of a circuit pattern to a surface of a sintered ceramic substrate.
2. Description of the Related Arts
A multilayer ceramic substrate has been proposed accompanied with the need demanding to reduce the size of the ceramic circuit substrate and to further enhance its density to be higher. Particularly a glass ceramic multilayer substrate that can be sintered at a low temperature ranging from 800.degree. to 1000.degree. C. has been highly expected to cope with the above-described need, which is allowed to contain low conductive resistance such as Ag, Cu or the like as an inner layer.
As an initial step of producing the above ceramic circuit substrate, via holes 921 are pierced through a plurality of ceramic green sheets 920 as FIG. 11 shows. Then the via hole 921 is filled with a conductor 951. A paste 952 for forming an inner layer pattern is printed on a surface of the ceramic green sheet 920. Those ceramic green sheets 920 are laminated and co-fired. As a result, a sintered ceramic substrate 92 having an inner layer pattern 95 and the conductive via hole 921 is obtained as FIG. 12 shows.
As FIG. 13 shows, a paste 932 for forming the circuit pattern is printed on a surface of the sintered ceramic substrate 92 for firing again. A circuit pattern 93 is formed on the surface of the sintered ceramic substrate 92, resulting in providing a ceramic circuit substrate 9 having a multilayer construction as shown in FIG. 14. The method of producing the ceramic circuit substrate as described above uses porous ceramic green sheets 920 as shown in FIG. 15 that serve to absorb solvent 959 contained in the paste 952 easily. When printing the paste 952 for forming the inner layer pattern to the above-featured ceramic green sheet 920, the ceramic green sheet 920 absorbs the solvent 959 contained in the paste 952. Therefore stain and bleed hardly occurs after printing, thus forming the inner layer pattern with a sharp and fine line.
On the contrary as shown in FIG. 16, it is difficult for the sintered ceramic substrate 92 obtained by co-firing the ceramic green sheets to absorb the solvent contained in the paste 932 for forming the circuit pattern sufficiently. In case of printing the paste 932 to the surface of the sintered ceramic substrate 92, most of the solvent cannot be absorbed thereby. As a result, the paste 932 that has been printed is likely to flow to the outside of a specified dimension 930 immediately after printing, resulting in a stain 939. Or the solvent may bleed on the surface of the paste 932, thus causing a bleed 938. Due to the above described stain or bleed, the fine line cannot be formed. Failing to form the fine line may obstruct the effort to reduce the size of the ceramic circuit substrate and to enhance its density to be higher.
Forming an outermost circuit pattern on a ceramic green sheet in the same manner as forming the inner layer pattern has been proposed. This process, however, causes variation in shrinkage among ceramic green sheets at firing. This deteriorates the accuracy for positioning the circuit pattern, obstructing the mount of a part to the circuit pattern.